01 -  Set air fryer to 375°F and preheat for 3 minutes. 
 02 -  Spread softened butter evenly on one side of every bread slice. 
 03 -  Place two bread slices, buttered side down, on a clean surface. Layer two cheese slices on each. Add optional tomatoes, caramelized onions, or bacon if desired. 
 04 -  Top with remaining bread slices, buttered side up, to complete the sandwiches. 
 05 -  Arrange sandwiches in a single layer in the air fryer basket. Cook in batches if necessary. 
 06 -  Air fry for 4 minutes. Carefully flip sandwiches, then air fry for an additional 3 to 4 minutes until bread is golden and cheese is melted. 
 07 -  Remove sandwiches and let rest for 1 to 2 minutes. Slice as preferred and serve hot.